Communication systems that utilize spread spectrum modulation and demodulation for security of transmission have been available. The article entitled "Spread Spectrum Modulation" by M. D. Egtvedt, of the text entitled "Electronics Engineers Handbook" published by McGraw-Hill in 1975 beginning at page 14-51 is exemplary of one such communication system. A spread spectrum is formed by modulating an RF signal with a pseudo-random noise (PRN) digitally coded signal. Typically, an RF transmitter includes a pseudo-random noise generator the output signal of which modulates the RF signal by a code of known sequence. A corresponding generator at a receiver, having an identical code sequence, generates a signal to demodulate the spread spectrum signal when the two codes are correlated by being in proper time synchronization. In such spread spectrum communication systems a threshold detector is employed to adjust the time relationship between the signals of the pseudo-random noise generators in the receiver and transmitter, respectively by a trial and error procedure which steps the time sequence of the pseudo-random noise code generator until correlation is detected between the received code and the receiver-generated code.
One well-known apparatus for providing threshold detection is known in the art as a fixed integration-time detector. Another threshold detector in the prior art is known as a sequential threshold detector which has certain performance advantages relative to the fixed integration-time device.
Sequential threshold detectors depend on the application of a very stable bias voltage signal to a differential amplifier for subtraction from the incoming signal after the PRN code signal is applied for demodulation. The difference signal is then applied to an integrator and comparator where it is first integrated and then compared with a negative threshold voltage. If the two PRN codes are in synchronization, the output voltage of the integrator will not exceed a selected negative threshold voltage during a given period of time, commonly referred to as the truncation time. If the PRN generator in the receiver is not synchronized to the PRN generator in the transmitter, the integrator output voltage exceeds the selected negative threshold voltage indicating that a lack of correlation exists between the two codes. When the integrator output voltage exceeds the selected negative threshold voltage, the integrator is reset for another attempt at acquisition with the receiver's PRN generated code shifted in time sequence for the new trial.
Important performance parameters for sequential threshold detectors are, probability of detection, probability of false alarm and median or mean dismissal time. The dismissal time is the time it takes the integrator output signal to reach a selected threshold voltage when a condition of noncorrelation is known to exist. These performance parameters are dependent upon the selected truncation time, threshold voltage, bias voltage, and upon the signal-to-noise ratio environment in which the receiver must operate. Typically, a sequential threshold detector circuit is designed so that the truncation time, threshold voltage, and bias voltage provide optimum performance parameters for a given minimum signal-to-noise ratio.
A prior art scheme for implementing sequential threshold detectors uses a highly stable, constant DC voltage as a threshold source and an accurate digital counter to determine truncation time. In this scheme, the bias voltage is established by sampling the incoming signal when correlation is known to be absent, and thereafter averaging and multiplying the sampled signal by a fixed gain to develop the bias voltage. However, such a scheme is susceptible to errors because of instability and inaccuracy of the bias voltage. Unfortunately, such instability and inaccuracy is difficult and expensive to avoid. In addition, conventional circuits that are used to subtract the bias voltage from the incoming signal, as well as circuits used for the integrator, develop DC offset voltages that create errors in the detector and, therefore, reduce performance from the designed optimum. Such DC offset voltages also occur in the bias voltage generating circuit within the detector and can become quite significant due to aging of components and changes in ambient temperature. In prior art sequential threshold detectors, reduced performance of the detector results from DC offset voltages. Consequently, more costly components and longer manufacturing time are required to achieve optimum performance despite component aging and ambient temperature variation.
The present invention utilizes a novel feedback circuit for selectively biasing or amplifying the incoming signal, to result in a substantially constant median dismissal time, at some optimized value. The result is a substantial improvement over prior art sequential detection circuits which substantially overcomes the aforementioned disadvantages of prior art sequential threshold detectors.
Although adaptive devices for receivers have been disclosed in the prior art, (see for example, U.S. Pat. No. 3,599,105 to Wear et al.) a sequential threshold detector that utilizes a feedback circuit for varying the bias or gain applied to the incoming signal to provide a substantially constant median dismissal time is not available. Other prior art detection devices are disclosed in U.S. Pat. No. 3,828,204 to Farnsworth and in U.S. Pat. No. 4,086,651 to Muir et al., but such devices do not utilize the novel feedback circuit of the present invention.